This invention relates to a method for controlling renewal of data in an erasable optical disc memory, such as magneto-optical disc memory, phase change type optical disc memory, etc. and in particular to a method for erasing information which overcomes the drawback that an overwrite operation is not possible for these memories which produces no problems at the moment of actual memory operations.
Magneto-optical disks, phase change type optical discs, etc. have been developed as optical disc memories for which information is erasable.
A prior art magneto-optical disc memory is described e.g. in Nippon Oyo Jiki Gakkaishi (Journal of the Japanese Applied Magnetism Society) Vol. 8, No. 5 (1984) pp. 345-349, "Actual State of the Magneto-Optical Disc Memory".
In such a type of erasable optical disc memory in order to renew (rewrite) data in a portion, where information has been already written, it is necessary to write data to be recorded after having effected an operation to erase the data in that portion, and therefore it has a drawback that overwrite, which can be effected for magnetic recording, is impossible. That is, in a prior art magneto-optical disc memory or phase change type optical disc memory, a write instruction is sent through an external interface together with a file name and data to be written. Then, the data in the directory track (disposed e.g. in an inner peripheral portion of the disc) on the disc is read-out. According to the result thus obtained, an operation to erase partially the directory data is effected at first and after that the directory data is renewed. Then, when there is found data having the same file name as that for the data to be written in the read-out directory data, the head is moved to the track including the sector corresponding to the file name in the data recording region, the data in that portion is erased, and then an operation to write the data in that portion is effected. In the case where data is read-out, read-out of the data in the directory track, movement of the head corresponding thereto and read-out of the data in the data record region relevant to the directory are effected.
Consequently, the erasable optical disc memory, such as the magneto-optical disc memory, the phase change type optical disc memory, etc., has a drawback that the cycle time for the write operation is longer than that for the magnetic disc memory. The reason therefor is due to the drawback that the erase and the write operation cannot be effected simultaneously, as described above. For example, for a magneto-optical disc memory, it is necessary to give it magnetic fields having different directions for writing and for erasing and at the same time to project laser light thereto. Therefore, if an operation to change the direction of the applied magnetic field in a very short time, e.g. in 50-100 ns, were possible, it would be possible to erase and write data simultaneously. However the inductance L of a coil generating the bias magnetic field is about 200-400 .mu.H, the current of the coil about 0.2-0.4 A and the bias magnetic field H.sub.B to be applied several hundreds Oe. The driving voltage E, which is necessary for inverting the magnetic field generated by the coil in a period of time .tau. to 50-100 ns is given by E=(I/.tau.)L . Using the values indicated above of L, I and .tau., the value of E is 500-2000 V. Since such a high voltage is necessary therefor, in practice, it is very difficult to overwrite data in a magneto-optical disc memory, i.e. to effect an erase and a write operation simultaneously.
On the other hand, in a phase change type optical disc memory, for effecting an erase operation, irradiation with laser light pulses is necessary using pulses having a width which is as long as several .mu.s and which is weaker than that necessary for a write operation. Consequently it is not possible to overwrite data in it, similarly to the magneto-optical disc memory.
As the result, for writing data in an erasable optical disc memory device it is necessary to perform the following 5 steps: read-out data in the directory track, erase data in the directory track, write data in the directory track, erase data in the data track, and write data to be written in the data track. Further, in the case of the magneto-optical disc memory, a rotation waiting time, during which the disc makes one turn, is necessary in order to permit the inversion of the bias magnetic field during a period of time between the erase and the write operations. Consequently, apart from the time necessary for displacement of the head between different tracks, even if the amount of data to be written is so small that it can be recorded in one track, the disk should make more than 6 turns, before the write operation is terminated. For a rotation speed of 3000 rpm of the disk, this waiting time is longer than 120 ms. Further, adding the time necessary for the displacement of the head (80 ms.times.2) thereto, the time necessary for the write operation is about 280 ms. For the phase change type optical disc memory, since the waiting time for inverting the bias magnetic field is not necessary and all the operations are terminated during 4 turns of the disk, the time necessary for the rotation of the disk is 80 ms. Therefore the time necessary for the write operation is about 240 ms.
Several methods for resolving the above problem have been proposed. Among them there is known a method, by which two heads are used. However, this method has drawbacks in that the device for realizing it is expensive and that its data rate is as low as 100 kb/sec.